[BUGFIX] Use correct timezone in DataMapperTest

The date/time generated by the datamapper is in UTC, so the test
must check for a date/time in that timezone too independent of the
configured timezone of the system.

Resolves: #75954
Releases: master, 7.6
Change-Id: I64f014b0d60b805bcbffd08dc50b924b925d022a
Reviewed-on: https://review.typo3.org/47945


Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>